1. Field of the Invention
The present invention relates the oxidation of carbon black with an aqueous mixture of hydrogen peroxide. More specifically, the present invention relates to an improved method for oxidizing carbon blacks, including rubber grade carbon blacks, with liquid hydrogen peroxide where the carbon blacks are efficiently oxidized in industrial quantities using standard carbon black production equipment and production rates and under conditions that produce no hazardous emissions and require no special handling.
2. General Background
Carbon black is the accepted generic name for a family of small particle size carbon pigments which are formed in the gas phase by the thermal decomposition of hydrocarbons. Carbon blacks are currently sold in the form of more than 100 different commercial grades which vary in their particle size, aggregate structure, porosity, and surface chemistry.
Historically, carbon blacks have been manufactured by different major processes including the lampblack process, the impingement process, the acetylene black process, the thermal black process, the gas furnace, and the oil furnace process. The process used to manufacture the carbon black is extremely important; indeed, the manufacturing process frequently has more of an impact on the product and its final characteristics than the raw material from which it is made.
The lampblack process is the oldest of all the processes and is now practically obsolete in the U.S. and most other locations. Lampblacks are made by burning aromatic oils such as naphthalene, anthracene, or creosote. Typically, the oil is burned in shallow pans with limited air supply and the carbon black formed is drawn off into settling areas and collected periodically. This is a low temperature operation and the particle size tends to be fairly large (about 70 to about 100 nm average diameter) and highly aggregated to produce a stringy chain structure.
Impingement (or "channel") process carbon blacks were first produced during the 1870's and reached their peak production in the 1920's and 1930's. In this process, natural gas (typically reinforced with a vapor of hot oil) was burned from slotted lava tips to produce fan-type flames in evenly spaced rows. These were set to impinge on channel irons on which the carbon black was deposited. During exposure to air on the hot channel irons, the carbon black becomes highly oxidized (e.g., 2.5 to 3.5% oxygen content). This high level of oxygen, obtained without resorting to a post treatment, is unique among the different carbon black processes. The combination of the natural gas feedstock and cooling without quench water also provides for a very low inorganic content. Unfortunately, the process is relatively inefficient in its use of increasingly expensive natural gas, and can pollute the atmosphere. Thus, it too has fallen into the same category as the lampblack process and is little used in the U.S. Carbon blacks with high oxygen surface functionality are now generally produced by post chemical oxidation of oil furnace products.
In the acetylene black process, carbon black is made from the thermal decomposition of acetylene gas. In one common embodiment, acetylene gas is introduced at the top of a cylindrical reactor which is several meters high. The reactor is preheated to about 800.degree. C. to decompose the acetylene. This is a rather violent exothermic reaction which produces temperatures up to about 3000.degree. C. Proceeding from the entry point of the acetylene gas down into the upper part of the furnace there is a temperature gradient of about 3000.degree. C. Carbon black formation most likely takes place in the 800 to 2000.degree. C. zone, followed by partial graphitization in the higher temperature region. Air enters into the bottom of the reactor and acts to combust the hydrogen that is formed. In addition, the air flow prolongs the residence time of the acetylene black aggregates in the hot zone of the reactor which eliminates any residual hydrocarbons and further unifies the highly graphitic microstructure of the final product. Acetylene black is also produced commercially by the thermal decomposition of acetylene gas in stainless steel retorts which are water cooled. Because of its highly graphitic surface, acetylene black exhibits low reactivity and contains a very low level of oxygen volatile. This type of carbon black is frequently used in conductive applications.
Thermal blacks are manufactured by the decomposition of natural gas or oil. They are made in the absence of air by means of a batch type process which is based on sets of dual furnaces (generators). The generators are lined with an open checker brickwork which is preheated prior to charging them with the gas or oil feedstock. The respective heat and make cycles commonly require a few minutes each. Following the make cycle, there is a one-minute steam purge to remove the carbon black, which is then water quenched, passed through the collection filter, and air conveyed to the beader, dryer, and bulk storage loading tank. Following the purge to remove the carbon black, air is passed through the system and carbon remaining on the walls of the generators is burned off to produce additional heat for the next make cycle. Thermal blacks are uniquely large in size (250 to 500 nm average particle diameter) and low in structure (aggregation) relative to all other types of carbon black. They are typically used in cross--linked polyethylene and in rubber applications requiring very high filler volume fractions, or in costly specialty polymers sensitive to degradation with other carbon black grades.
Most of the carbon black grades available today are made by the oil furnace process, which involves the decomposition of highly aromatic oil feedstock in a heated reactor. This is a very rapid, continuous process in which the oil is converted to carbon black aggregates in a few milliseconds. The feedstock oil from storage tanks is injected into the reactor which is heated continuously with a mixture of fuel (oil or gas) and air. The carbon black is water-quenched immediately after formation and then further cooled as it passes through a heat exchanger and on through the bag filter and into the beaders, dryer, and storage tank.
It has long been recognized that oxidized carbon blacks (carbon blacks treated so as to have oxygen--containing functional groups at the surface) feature characteristics which are important to specific applications. For example, in paint, ink, toner, and coatings applications, oxidized carbon blacks provide improved wettability and rheology, characteristics which are important in these applications.
More recently oxidized rubber carbon blacks have been shown to provide enhanced filler-elastomer interaction in crosslinked polymers resulting in improved filler-elastomer interaction (De Trano et al., Bridgestone Corp., U.S. Pat. No. 5,248,722). This improved interaction leads to desireable dynamic properties such as higher elastic modulus and reduced tan delta or hysteresis. Reductions in hysteresis are of utmost concern for tire manufacturers since this property is directly related to the rolling resistance of the tire. It is well known to those skilled in the art that rolling resistance can be predicted from measurements of tan delta at low dynamic strain amplitudes around one percent at about 50 degrees centrigrade.
Obtaining the improved filler-elastomer interaction from oxidized rubber carbon blacks requires chemical interaction with the polymer, such as, might be obtained from a functionalized elastomer in which some or all of the polymer ends contain chemically modified terminal groups.
Previous commercial methods for oxidizing carbon black have included ozone in the gaseous phase, and nitric acid in the liquid phase. Both methods have been used for oxidizing specialty carbon blacks. These methods are inefficient for oxidizing rubber grade carbon blacks because rubber grade carbon blacks are produced at much higher rates. Also the ozone and nitric acid methods both potentially emit noxious gases and require costly corrosion-proof equipment beyond what is normally utilized in carbon black production. A better method to oxidize rubber grade carbon black would entail no hazardous emissions, no large capital expenditure for equipment, ability to oxidize at rubber grade carbon black production rates, and economical cost.
Gaseous hydrogen peroxide has also been used to oxidize carbon blacks (See, for example, U.S. Pat. No. 3,279,935). However, unlike the instant invention which uses liquid hydrogen peroxide solution, the use of gaseous hydrogen peroxide to oxidize carbon blacks poses significant safety risks and expenses, and is not well suited for industrial use to oxidize rubber grade carbon blacks. For example, by using gas phase hydrogen peroxde, the '935 method produces dangerous gases and must utilize fluid bed or Roto-Louvre driers, rather than conventional carbon black process equipment like the instant invention.
A Russian paper has disclosed the use of hydrogen peroxide in a liquid phase to oxidize carbon black (Lezhnez, N. N., et al., Proizv. Shin Rti lati 1974 No. 11, 1974, P. 10-13). However, unlike the instant invention where the hydrogen peroxide/carbon black mixture is dried, and/or heated for drying, and where a drying step is clearly a necessary step in order to oxidize the carbon black and no pre-drying/heating mixing time is required, the Russian paper teaches that a stirring time of 50-70 minutes and a cooling time at room temperature of 45 minutes are necessary for oxidation, and that drying and heating the mixture are not steps in the oxidation process. The paper reports that "the hydrogen peroxide [breaks] down completely" during the pre-heating mixing and cooling. Therefore, the paper teaches that the oxidation of the carbon black is considered to be completed prior to, and regardless of whether, any drying step, or heating to effect a drying step, is performed subsequent to mixing the carbon black with the liquid hydrogen peroxide.
However, contrary to the report of the Russian reference, the data presented herein show that in order to efficiently and effectively oxidize carbon black with liquid hydrogen peroxide, it is necessary that the hydrogen peroxide/carbon black mixture be dried and/or heated to a temperature of at least 105.degree. C. to effect drying. Further, it is herein shown that the pre-heating mixing/cooling period can be kept to a minimum; merely the time necessary to mix the reagents.
Hence, the data herein present the first efficient and effective use of liquid hydrogen peroxide to oxidize carbon black.
Further, the present invention presents the first process for efficiently and effectively oxidizing carbon black that entails no hazardous emissions, no large capital expenditure for special and costly equipment, and the ability to oxidize at rubber grade carbon black production rates, resulting in economical production costs.